In order to manufacture a vessel according to the blowing technique, we begin by heating a preform (whether entailing a preform or an intermediary vessel that has undergone a first blowing operation from a preform) at a temperature greater than the glass transition temperature of the material of the preform. The preform is then introduced into a mould, then the blowing of the preform is carried out by injecting therein a gas (such as air) under high pressure (generally greater than 30 bars). The stretch blow-moulding technique consists, other than the blowing, in stretching the preform using a sliding rod, in order in particular to minimise the offset of the vessel and to render uniform as much as possible the distribution of the material.
In order to prevent the preform from bursting during the blowing operation, prior to the blowing, a pre-blowing operation is generally provided, during which a gas under reduced pressure (generally between 5 and 16 bars) is injected into the preform. It is during the pre-blowing operation that is carried out the stretching of the preform, with the blowing intervening only afterwards.
In a context of industrial production, where manufacturing rates reach several tens of thousands of vessels per hour and per machine (a machine generally comprises several moulds mounted on a rotating carrousel), the duration of the method for producing a vessel, between the moment the preform is introduced into the mould and the moment the formed vessel is ejected, is of a few seconds. The duration of the pre-blowing is only of a few tenths of a second.
The constant desire of industrialists to reduce the quantity of material used for each vessel, combined with the velocity of the method, makes the manufacture delicate and can lead to substantial scrap rates. Among the main defects observed on the vessels is a poor distribution of the material. The difficulty that exists in combining saving material and the increase in the rates has led the industrialists to become more interested in the unfolding of the method by monitoring the variations in certain parameters over time, the main one being the pressure inside the preform, combined with the stretching speed.
Certain entities, such as Du Pont De Nemours in its American patent U.S. Pat. No. 4,042,657, suggest that there is a correlation between the pressure variations in the preform and the final form of the vessel, and recommend rejecting the vessels of which the pressure curve does not comply with a standard curve. If the proposal to reject the non-compliant vessels makes it possible to improve the overall quality of production, it puts a strain however on productivity and, in the hypothesis where the rejected vessels would not be recycled, leads to a wasting of material although it is desired to save it.